Trying to get confidence intervals on a fixed effect in a mixed model fitted with the lmer function in package lme4 in R. I can get distributions of beta using "mcmcsamp" function and estimate a confidence interval from that, but I can also do it using "sim" in the package arm. While they both give me similar results, I'm curious what the difference is. In neither case do I have a good grasp of what's going on 'under the hood' - the arm package documentation just states that it provides "posterior simulations". Do the two methods have different qualities or are they essentially the same? Thanks!